Ryan said that the coalition fighting the Islamic State group, the USA started "the process of our deliberate withdrawal from Syria". "Out of concern for operational security, we will not discuss specific timelines, locations or troop movements".
The Syrian Observatory for Human Rights said a convoy of about 10 armoured vehicles and some trucks pulled out from the north-eastern town of Rmeilan on Thursday night and crossed the border into Iraq.
There are 2,000 US troops in Syria and while the military has orders to follow the president's decision to withdraw, they have no timelines attached for how long it will take.
National Security Advisor John Bolton appeared to contradict the White House Sunday when he said the USA withdrawal from Syria would not be immediate, but the Pentagon announced Thursday that without receiving any new direction, it's moving forward with the mission.
U.S. Secretary of State Mike Pompeo, who has been touring the Middle East this week to reassure allies of Washington's commitment to regional security, said on Thursday the withdrawal would not be scuppered despite the Turkish threats.
"This is the first such pullout of American forces since the U.S. president's announcement" of a full troop withdrawal from Syria last month, he said.
The fact that Bolton laid the demand for protection of Kurdish fighters as a condition for United States withdrawal from Syria sparked Erdogan's anger.
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America "will use diplomacy and work with our partners to expel every last Iranian boot" from Syria and bolster efforts "to bring peace and stability to the long-suffering Syrian people", he said during a speech in Egypt. He stressed that the fight against ISIS would continue, but he did no repeat the need for the Kurds to be protected, or mention Bolton's previous vow to curb Iran's presence in Syria.
The removal of the equipment in recent days was first reported by CNN, which quoted an administration official with direct knowledge of the operation as saying it signaled the beginning of U.S. withdrawal from the Middle Eastern country.
US forces' vehicles and structures are seen on the outskirts of the northern Syrian town of Manbij on December 26, 2018.
"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency", Trump tweeted that day, adding that USA troops would leave in the next 30 days.
While the USA military pullout is now officially underway it has made a slow start. The latter will no doubt prove the more hard objective to meet. Turkey views the Kurdish YPG militia, which forms the core of the SDF force, as a national security threat.
Syrian Kurds demonstrate against threats from Turkey to carry out a fresh offensive following the USA decision to withdraw their troops in the Syrian Kurdish-majority city Qamishli, on December 28, 2018.
Shortly after Trump's announcement, some USA officials began to walk it back.
In their Wednesday meeting, Trump, Shanahan and Dunford were also set to begin formulating a more fine-tuned withdrawal plan, according to several defense and administration officials.